motion tracking timeline colors
The blueish line means that the frames have been prefetched or cached to RAM. Prefetching will give you better performance and playback, at the expense of available RAM.

Uncached frames are being read and decoded directly from the disk file.
(Form the Blender Manual)

Prefetch P
Fills cache with frames. As many frames as fits into cache are load form the drive. This allows to fill in the cache as fast as possible when you really need to track something, but this keeps CPU and drive bandwidth idle if you’ve got Clip editor opened but not actually interacting with it.

It is therefore recommended that you prefetch (cache) all of the frames that you are going to be working with to RAM. To do this load the video file on to the movie clip editor and press P to prefetch (or you can simply play back the video once and it will be cached).
By default blender will only use 1GB of RAM to cache frames. You can change that number depending on the amount of memory on your system. To do that open the User prefences (Ctrl + Alt + U) and on the System tab you'll see the options for the prefetch and cache limits.

Dark purple starts where the tracking is not reliable and has larger errors
If you see red-magenta on the timeline that means that Some Data Failed to Reconstruct, which means that the tracker information is missing or not reliable enough to reconstruct the 3d scene.
Read: Some Data Failed to Reconstruct, Motion Tracking
Recommended reading: How can I get better results when doing camera motion tracking?